A Drupal Multi-site is essentially a site-let that shares code with a main site but has its own database. Multi-sites are generally easier to maintain because the code is maintained in the parent project -- the main thing that needs attention are database updates.
Terms & Conditions
a. Multi-sites must have a parent site. Freelock is developing several standard template-driven sites that can qualify -- but generally multi-sites are for customers who want a copy of their main site to use the same concept for a different audience, with no shared content.
b. Multi-sites under this plan do not have a test instance. Updates are rolled out automatically, with very limited testing.
c. Customizations to multi-sites that break during updates may lead to either disabling of the broken module, or billable hours to fix.
d. A multi-site has its own database, or its own prefix in a shared database, completely independent of the database of the parent site.
e. Each multi-site in use bears its own additional charge.